1. The Field of the Invention
The present invention relates to the design and method of use for an implant to help realign angular and rotational deformities in long bones in patients with active growth plates.
2. Related Technology
As a result of congenital deformation, traumatic injury or other causes, long bones such as the femur, tibia and humerus may grow out of alignment, causing deformity of the limb and biomechanical abnormalities. While some deformities are asymptomatic or may resolve spontaneously, it is often necessary to intervene surgically to realign these limbs. For the patients requiring surgical intervention, both osteotomy with realignment of the bone and epiphyseal stapling are currently accepted methods of treatment.
One common method of surgical bone realignment is by means of an osteotomy, or cutting of the bone, followed by realignment of the bone. In some procedures the bone is cut laterally, transverse to the longitudinal axis of the bone. Then the bone is realigned. A bone graft is then placed in the resulting wedge space. The bone and the bone graft are stabilized by orthopedic fragment fixation implants such as screws and bone plates. In an alternative osteotomy procedure, a bone wedge is removed. The bone is realigned, and similar implants are used to secure the bone. A third method of deformity correction via osteotomy is to first cut the bone, then apply an external frame attached to pins drilled through the skin and into the bone. By adjusting the frame, either intraoperatively or postoperatively, the bone is straightened.
Because osteotomy methods require a relatively large incision to create bone cuts, they are relatively invasive; they disrupt the adjacent musculature and may pose a risk to the neurovascular structures. An additional disadvantage of these procedures is the potential risk of damage to the growth plate, resulting in the disruption of healthy limb growth. Consequently, this procedure may be reserved for bone alignment in skeletally mature patients in whom the growth plates are no longer active.
One less invasive method of bone alignment involves the placement of constraining implants such as staples around the growth plate of the bone to restrict bone growth at the implant site and allow the bone to grow on the opposite side. First conceived in 1945 by Dr. Walter Blount, this method is known as epiphyseal stapling. Typically epiphyseal stapling is more applicable in young pediatric patients and adolescents with active growth plates. A staple is placed on the convex side of an angular deformity. Since the bone is free to grow on the concave side of the deformity, the bone tends to grow on the unstapled side, causing the bone to realign over time. Once the bone is aligned, the constraining implants are typically removed.
As long as the growth plate is not disturbed, this type of intervention is generally successful. However, the procedure must be done during the time that the bone is still growing, and the physiodynamics of the physis (growth plate) must not be disturbed. With proper preoperative planning and placement of the implants, the surgeon can use the implants to slowly guide the bone back into alignment.
The implants currently used in epiphyseal stapling procedures are generally U-shaped, rigid staples. The general design has essentially remained the same as those devised by Blount in the 1940's. Since these implants are rigid, they act as three-dimensional constraints prohibiting expansion of the growth plate. They are not designed to allow flexibility or rotation of the staple legs with the bone sections as the bone is realigned. Due to the constraints of these staple implants, the planning associated with the placement of the implants is overly complicated. Consequently, the surgeon must not only determine where to position the implant across the physis, but also must account for the added variables of implant stiffness, implant strength and bone-implant interface rupture.
The force associated with bone growth causes bending of these implants proportionate to their stiffness. Depending on the strength of the implant, these loads could eventually cause the implants to fracture under the force of bone realignment. This can make them difficult or impossible to remove. These same forces can also cause the implants to deform, weakening the bone-to-implant interface. This weakening may result in migration of the implant out of the bone, risking damage to the adjacent soft tissues and failure of the procedure.